I am going from US to Changsha and staying for a few days.

Then going from Changsha to Bangkok for a week, then back to Changsha and then US. I want to stay in Changsha 1 week each time. I am planning to get a multi-entry visa and I will have airline tickets and hotel confirmation for the entire trip. I just want to know if I will have any problems? How often can I use the multi-entry visa? Will I have to go to the embassy to get a new visa?